1. Field of the Invention
This invention relates to apparatus for leveling flowable materials such as concrete, asphalt, dirt and gravel in which an elongated leveling member is maintained in alignment with a desired leveling plane by a laser alignment system. In particular, it relates to an improvement in such a system having a pair of laser beam detectors on opposite ends of the elongated leveling member so that the system can maintain leveling accuracy when one of the detectors is temporarily blocked from receiving the laser bean such as by a support column, a truck, personnel or other obstruction.
2. Background Information
The invention has particular application to concrete screeds, but has application to other apparatus for leveling flowable materials such as, for instance, graders.
The concrete screed is a device used when pouring and leveling (i.e., screeding) large concrete floors, such as used in large warehouse buildings, department stores and other large area construction projects as well as large outdoor concrete areas. Examples of concrete screeds are provided in U.S. Pat. Nos. 4,655,633 and 4,930,935. These machines consist of a self propelled, rotating turret supporting a large telescoping or extendable boom, which can typically extend 20 feet from the turret. At the end of the boom is a self adjusting screed head, typically about 13 feet wide, which can automatically maintain a level position regardless of the machine's position. During the pouring of a large concrete floor, trucks deliver the concrete and discharge it on the ground. The screed head is positioned at the head of the freshly discharged concrete at exactly the desired horizontal height. The screed head is then smoothly pulled toward the turret by the boom, thereby evenly leveling and consolidating the concrete at exactly the desired horizontal height. The screed head is then repositioned to an adjacent location where fresh concrete has been placed by a truck. Again, the screed head is retracted toward the turret creating another section of smooth, (screeded) concrete.
One of the important features of these prior art concrete screeds is that they produce an extremely flat, level concrete floor in a short time-period. Each of the individually screeded sections matches the height of the adjacent sections with precision. In order to accomplish this, an automatic laser alignment system is provided. A laser projector is first installed outside the perimeter of the area to be poured. The laser projector generates a beam which is swept in a horizontal plane to provide an extremely fiat leveling plane, normally several feet above the ground. Sensors responsive to the laser beam are located on posts at both ends of the moveable screed head. These sensors monitor the beam location relative to the end posts and therefore can transmit electrical signals indicating how much the leveling plane is above or below the desired location on the end posts. Vertical actuators, such as hydraulic cylinders, are provided for both ends of the screed head. These vertical actuators allow each end of the screed head to be vertically moved so that the respective ends are kept within a very small tolerance of a desired vertical position as the screed head pulls the concrete toward the turret. A controller provides automatic control of the screed end positions, and therefore, screed height and levelness, during the pulling of the concrete. For example, if the controller detects that the laser beam is moving down on a screed end sensor it provides signals to the actuator (such as to solenoid valves for the hydraulic cylinder) for that end to move the screed end down until the beam is centered again on the sensor. As the screed is moved from position to position, the screed head always reestablishes and maintains the correct vertical location of both ends by means of the laser alignment system.
The commercial embodiment of the concrete screed described in the above patents is known as the LASER SCREED (trademark Somero Enterprises, Inc.). The LASER SCREED incorporates a feature which provides frequent control corrections for the screed end height. The self-propelled frame has stabilizer legs by which it can be tilted so that the boom retracts along an upward 2% grade (i.e., the boom is angled upwards from its free end to the turret). As the boom end is pulled toward the turret, it is gradually rising in elevation. However, the laser sensors on the screed head ends are rapidly and continually sensing the height of the screed end posts. When the screed end post height increases by the very small margins permitted by the machine (i.e., 1/8" increase), the laser sensors send a signal to the central processing unit to adjust the screed end post height back down to the desired elevation. In this manner, each side of the screed head is independently and automatically controlled to stay within a very narrow elevation tolerance band. This provides for a practically seamless control of concrete height between adjacent sections that are pulled, plus providing a very flat, level surface within the pulled section. The operator merely has to command the screed boom to be retracted and the concrete is automatically screeded to the correct height, blending with a height of adjacent concrete sections.
While the laser screed works very well, there is one problem which has remained unsolved during the eight (8) years since the introduction of the LASER SCREED. The laser sensors can only function if they have an unobstructed view of an alignment source. If the laser beam is broken by an object, such as a building vertical support column, then the sensor will not provide a signal to the controller, and the affected screed end will not automatically adjust its height. Unfortunately, most building sites have a dozen or even several dozen, vertical support columns which are in place when the concrete floor is poured. As the screed is moved around the floor, pulling the numerous section of concrete, the vertical support columns frequently block the laser beam from hitting the sensors during part of each pull. This interference is called "column block" and it disrupts the automatic operation of the laser screed. Without a control signal, the controller unit does not know how to adjust the screed end. Therefore, an instrument light is provided to inform the operator that there is column block on the affected sensor. The operator must immediately assume manual height control of the affected screed end when the column block light is illuminated. The boom continues to pull toward the turret at its set speed. Because of the 2% upward grade established for the boom, the height of the screed end must somehow be adjusted to prevent the screed end from also rising at the 2% grade and therefore ruining the floor flatness. The operator must manually control the screed end height and estimate the correct height corrections as the boom is retracted until the sensor moves into a position where it is unblocked. At this point, the automatic system becomes active again. Unless the manual corrections were made reasonably accurately, the screed end is abruptly moved to its correct height. This corrective movement can cause an unwanted wave or disruption in the floor surface. It should be noted that while the boom is retracting, the operator is often rotating the turret fight or left in response to obstacles (such as the vertical columns) and is checking around the machine to make sure that none of the numerous personnel or their equipment are getting too close to the machine. It is therefore very undesirable to also require the operator to continually monitor whether column block exists. In spite of these unwanted characteristics, there has been no solution to the problem of "column block" up to this point. It has also been found that in some jobs, "column block" is more frequently caused by the trucks delivering the concrete and other equipment rather than the vertical columns. Thus, this provides even more incentive to find a solution to the problem.
There is a need, therefore, for an improved laser aligned apparatus for leveling flowable materials, and in particular for concrete screeds.
There is a specific need for such improved apparatus which can maintain floor levelness when one of the sensors becomes blinded by an obstruction.
There is an additional need for such improved apparatus which responds automatically to blinding of one of the laser detectors.